In microscopic imaging, efforts are continually being made to achieve higher resolution microscopic images. For example, in the case of scanning probe techniques, the quest for high resolution has traditionally been addressed by finding ways to reduce the probe size while minimizing the corresponding reduction of probe current so as not to encounter serious signal to noise limitations, since the signals measured are always directly proportional to the probe current. In scanning electron microscopy (SEM), for instance, the major ways to accomplish this has been through a combination of brighter sources, such as the use of field emission and Schottky sources, as well as improvements in electron optical design. Similar developments are taking place in other techniques such as scanning transmission electron microscopy (STEM), auger electron microscopy, focused ion beam (FIB), secondary ion mass spectrometry (SIMS), Rutherford backscattering (RBS), proton induced x-ray emission (PIXE), and other x-ray probing techniques including x-ray photoelectron spectroscopy (XPS).
All of these techniques, both scanning and non-scanning, often share the common goals of high spatial resolution, distinguishable levels of contrast and low signal to noise ratios. All of these factors are critical to obtaining useful images. For example, with little or no contrast, even when high resolution is possible, fine detail in an image will not be seen. Similarly, if the signal is very weak, even when high resolution is possible and the contrast is adequate, then noise may still limit the ability to obtain high resolution images of discernable features.
Many current developments in the scanning probe techniques have concentrated on probe size minimization coupled with attempts to choose experimental conditions such that the signal measured comes from a pixel corresponding in size to the probe size. However, because of the strong relationships that exist between resolution, contrast and signal to noise ratio in scanning probe techniques, reducing probe size becomes more prohibitive. Microscopes with extremely small probe sizes are expensive and require considerable care and expertise to maintain due to instrument design and environmental considerations such as mechanical vibrations, contamination, stray fields, and thermal variations, which can all undermine and limit the quality of results. A technique is needed that can provide accurate signal values for resolutions higher than those limited by probe size.